The human H.sub.2 receptor was first identified by Black et al Nature (1972), 236, 385-390. This was followed by the demonstration of the receptor in the mammalian brain by Baudry et al (1975) Nature 253, 362-363, and Haas and Bucher (1975) Nature 255, 634-635. Gantz et al (1991) Biochem. Biophys. Res. Comm. 178,3, 1386-1392 have recently identified the sequence of a human H.sub.2 receptor cDNA from gastric parietal cells by using the polymerase chain reaction (PCR) and degenerated oligonucleotide primers whose sequence was obtained from the canine H.sub.2 receptor previously cloned by this group, Gantz et al (1991) Proc. Nat. Acad. Sci. USA 88, 429-433. This sequence was characterised as an intronless gene encoding a typical seven transmembrane domain aminergic receptor protein.
The receptor is coupled to heterotrimeric GTPases (G proteins), but differs from other monoamine receptors in this G protein coupled superfamily in several respects. The human gastric H.sub.2 receptor is shorter than most other receptors in this class (359 amino acids) and lacks the two serine residues in the fifth transmembrane region (TM5). There exists instead an aspartate and a threonine residue, so far unique in this region. These two residues may be important for binding with the nitrogen atoms of the imidazole ring of histamine as suggested by Birdsall (1991) Trends in Pharmacological Sci. Jan, 12, 9-10.
Histamine is a natural constituent of many organs and tissues including the gastrointestinal tract, the immune system and the brain, Green et al (1987) Agents and Actions 22, 1-15. It is a central neurotransmitter in the brain and is formed in the posterior hypothalamus from exogenous histidine by histidine decarboxylase (HDC). It is subsequently metabolised by histamine methyltransferase (HMT), Prell et al (1986) Ann. Rev. Neurosci. 9, 209-254. The cell bodies and neuronal pathways for histamine have been mapped in the human brain using immunocytochemistry by Panula et al (1990) Neuroscience 34, 127-132. Its cells project from the tuberomamillary nucleus of the posterior hypothalamus to almost every region of the brain. There are three known histamine receptors; H.sub.1, H.sub.2 and H.sub.3, the latter functioning as an autoreceptor. The H.sub.2 receptor specifically has been localised in the human brain by Traiffort et al (1992) J. of Neurochem. 59, 1, 290-299. using receptor autoradiography.
Histamine is known to have significant effects in the central nervous system (CNS). It has been implicated in the CNS mediated mechanisms of arousal ever since the sedating effect of H.sub.1 receptor antagonists (eg,. chlorpheniramine, chloropromazine) had been noticed clinically. The use of H.sub.2 receptor antagonists in the human brain however, has shown, that these compounds, unlike those acting on the H.sub.1 receptor, do not produce any effect on psychomotor functioning, or a subjective feeling of sedation or arousal in healthy subjects, White et al (1988) Psychopharmacology 95, 1-14. Some H.sub.2 receptor antagonists (eg. cimetidine) are known to cause confusion in elderly or severely medically ill patients, perhaps in part due to a co-existing anti-cholinergic effect. H.sub.1 and H.sub.2 receptor antagonists in large doses have been reported to cause hallucinations, Csillag et al (1973) Med. J. Aust. 1, 653-654, Argawal (1978) J. Am. Med. Assoc. 240, 214. Animal studies have shown that histamine applied directly to the hippocampus, where there is the highest level of activity of the H.sub.2 receptor, will induce psychomotor withdrawal and decreased exploratory behaviour. The above evidence has led to the conclusion that H.sub.1 receptor systems are excitatory in the terms of arousal and motivated behaviour whilst H.sub.2 receptor systems are inhibitory in this respect, Alvarez and Banzan (1985) Physiol. and Beh. 34, 661-664 and (1986) Physiol. and Beh. 37, 39-45, White et al (1988) supra.
The H.sub.2 receptor is a site of action of various compounds used in the treatment of psychiatric disorders eg. amitriptyline and mianserin, Traiffort et al (1992) supra. Kaminsky et al (1990) The Lancet 335, 1351-1352 and (1991) Schizophrenia Bull. 4, 318-319 have reported the successful response of patients with chronic, predominantly negative type schizophrenia, to the highly specific H.sub.2 receptor antagonist famotidine. For example in one patient there was a substantial amelioration of the deficit symptoms of schizophrenia (eg apathy, social withdrawal, and blunted affect) while on famotidine, relapse in these symptoms on withdrawal, and improvement on re-institution of this drug, Kaminski, U.S. Pat. Nos. 5070101 and 5177081. Prell et al (1992) Abstract, part 1, 199.6 Soc. for Neurosci. Annual Meeting, Anaheim Calif. have shown substantially raised levels of N-tele-methyl histamine, a metabolite of histamine in the cerebrospinal fluid of patients with schizophrenia which correlates with those patients with the occurrence of negative symptoms of this disorder assessed using the Psychiatric Symptoms Assessment Scale. These levels were not significantly different between patients free from medication and those on neuroleptic therapy. It is therefore postulated that there is an increase in histaminergic activity in patients with chronic schizophrenia.
The disclosures of all the above mentioned publications are incorporated herein by reference for all purposes.
Additionally, histamine, acting via its receptors, including the H.sub.2 receptor, is believed to be critically involved in a number of diseases of organs other than the brain; these include peptic ulceration, allergic reactions, including asthma, immune-mediated disorders, and possibly some tumours.
The histamine H.sub.2 receptor is one of many receptors in the body. Compounds used to treat many diseases work by activating a receptor or inhibiting the action if its natural ligand. Variations in receptors amongst the population are known to be caused by allelic variation and this variation, can alter the response of a disease to a drug amongst patients. An example of this would be the response to clozapine, used to treat, schizophrenia associated with allelic variation in the 5-HT.sub.2A receptor demonstrated by Arranz el al (1995) Lancet, 346(8970), 281-282.